Conventionally, a cylinder head cover is provided with a spark plug fitting hole for inserting a plug tube in which a spark plug is housed. Further, a seal member is arranged between the inner circumferential surface of the spark plug fitting hole and the outer circumferential surface of the plug tube, wherein sealing with airtightness and liquid tightness is made between the cylinder head cover and the plug tube.
For example, Patent Document 1 discloses an invention wherein there are provided a cylinder head cover having a flange portion protruding from the lower part of the inner circumferential surface of a spark plug fitting hole inward along the axial direction, a plug tube inserted in the spark plug fitting hole, and an annular seal member disposed between the cylinder head cover and the plug tube, pressure fitted inside the spark plug fitting hole, and supported by and in contact with the upper surface of the flange portion.